1. Field of the Invention
An aspect of the present invention relates to a battery, and, more particularly, to a pouch-type battery using a pouch as an external case.
2. Description of the Related Art
Generally, a lithium secondary battery employs a non-aqueous electrolyte due to the reactivity of lithium with water. The non-aqueous electrolyte may be a solid polymer containing a lithium salt or a liquid in which a lithium salt is dissociated in an organic solvent. Lithium secondary batteries may be classified as either a lithium metal battery and a lithium ion battery, which use liquid electrolytes, or a lithium ion polymer battery, which uses a polymer electrolyte, depending upon types of the electrolyte employed by the batteries.
A problem of leakage of an organic electrolyte can occur in a gel-type lithium ion polymer battery containing an organic electrolyte, while the problem does not occur in a solid-type lithium ion polymer battery. This leakage may be prevented by a relatively simple operating method for the lithium ion polymer battery, in comparison with that of a lithium ion battery using the liquid electrolyte. For example, in the lithium ion polymer battery, a multi-layered pouch including a metal foil and one or more polymer membranes, which cover top and bottom surfaces of the metal foil, are used instead of a metal can that is used in the lithium ion battery.
When the multi-layered pouch is used, it is possible to reduce the weight of the battery, to reduce the thickness of the battery, and to relatively freely change the shape of the battery, in comparison with those cases in which the metal can is used.
FIG. 1 is a perspective view of a conventional pouch-type lithium secondary battery illustrating a status in which a pouch is not sealed. As shown in FIG. 1, the conventional pouch-type lithium secondary battery includes an electrode assembly 30 and a pouch 20 to receive the electrode assembly.
With reference to FIG. 1, in a general method of assembling a pouch-type lithium secondary battery, a middle portion of an approximately rectangular pouch membrane is folded to form a front side 21 and a rear side 22 of the pouch. A groove 223, in which the electrode assembly 30 is accommodated, is formed on the rear side 22 by a process, such as a press working process. The indented portion 223 formed in this manner makes an installation of the electrode assembly 30 in post-processes possible, thereby making a performance of the assembling processes relatively easy. In addition, owing to the presence of the groove 223, a sealing part of the pouch 20 around the groove 223 may be arranged, thereby allowing for a compact formation of the pouch.
A multi-layered film, which is formed by sequentially stacking a positive electrode 31, a separator 33, and a negative electrode 35, is wound in a spiral form to form the conventional electrode assembly 30 to form an arrangement resembling a jelly roll. When the jelly roll is formed by winding the multi-layered film, a separator is added to an external electrode surface that is exposed from the jelly roll or an internal electrode surface to prevent an occurrence of a short circuit between the positive electrode 31 and the negative electrode 36. The formed jelly roll is disposed in the groove 223 of the rear side 22, and the front and rear sides 21 and 22 of the pouch 20 are heated and pressed to form a bare cell of a battery while the front side 21 of the pouch 20 and a flange part 225, which is flange shaped, of the rear side 22 of the pouch 20 are brought into tight contact with each other.
Electrode taps 37 and 38 or electrode leads to electrically connect the positive and negative electrodes 31 and 35 of the electrode assembly 30 to an external circuit outside the pouch 20 are respectively formed in one side of the positive electrode 31 and one side of the negative electrode 35. These electrode taps 37 and 38 are formed to be projected from the jelly roll in the direction perpendicular to the winding direction of the jelly roll and are drawn out through one side of the pouch 20 to be sealed.
In the process of sealing the pouch 20, a predetermined ingredient may be added to a surface of the polymer membrane to reinforce the bonding between the polymer membrane inside the pouch 20 and a metal constituting the electrode taps 37 and 38. In addition, an insulating tape 39 may be further included to prevent an occurrence of a short circuit between the electrode taps 37 and 38 and the exterior frame of the pouch 20 before the pouch is sealed.
Accessories or structures such as a protective circuit module (PCM) (not shown) or a positive temperature coefficient (PTC) (not shown) may be attached to the bare cell, of which the pouch has been sealed, to form a core cell. Thereafter, the core cell is inserted into a hard case to form a hard pack battery. Recently, in order to save space of the battery and to simplify the assembling process, a type of a battery has been developed, in which the external shape thereof is formed by closing both ends of a pouch of the battery in the longitudinal direction of the pouch and in which a circuit board and a protection member are attached to the pouch with a hot melt resin. In this type of battery a hard case is not required.
FIG. 2 is a perspective view of a conventional pouch-type lithium secondary battery in a state in which edges of two sides of a bare cell that are opposed to each other from which the electrode taps are not drawn out, are folded. FIG. 3 is an enlarged cross-sectional view of the conventional pouch-type lithium secondary battery taken along line A-A in FIG. 2.
When a hard pack is formed without a folding of sealing parts 25 of the bare cell, or where, more particularly, the sealing parts 25 of two opposite sides from which the electrode taps 37 and 38 are not drawn out, an unnecessary space corresponding to the width of these portions is formed in the hard case. Accordingly, while the core pack is formed of the bare cell, both sealing parts 25 are folded toward the groove 223 in which the electrode assembly is disposed. When the pouch forms an external shape of the battery without an insertion of the pouch into the hard case, the sealing parts 25 of both sides of the pouch are folded to decrease the entire width of the battery in the same way as is described above.
Accordingly, in the processes of assembling the conventional pouch, the groove 223 is first formed on the rear side 22 thereof. Thereafter, a flange part, which is an edge portion around the groove 223, and an edge portion of the front side, which becomes a cover of the groove 223, are welded to each other and sealed. Thereafter, the sealing parts 25, on the opposite sides of the conventional pouch in the widthwise direction are bent toward the groove 223.
Recently, battery makers have been required to provide that two sides of the pouch are formed in a curved shape due to a problem in a design of a pouch-type battery or an electric or electronic device such as a cellular phone that is fitted with the pouch-type battery. Here, since the electrode assembly of the pouch-type battery has an elliptic or track shape and not an angled shape, when the sides of the pouch-type battery are formed in the curved shape, the electrode assembly fits inside the pouch-type battery substantially without any empty space. Accordingly, improvement in capacity-to-volume ratio of the battery may be expected.
However, in the case of the conventional pouch in which the groove is formed, a portion forming the side walls of the groove and a flange part around the groove are approximately perpendicular to each other in a deep-drawing process to form the groove. In other words, angled corners are formed. When both sealing parts are bent toward the groove after the sealing, the bent portions form sharp corners due to the sharp corners which have already been formed in the rear side of the conventional pouch. Thus, it is difficult to form the sides of the pouch into a curved surfaces. Thus, the ratio of capacity to volume of the battery is decreased.
In addition, the entire width of the bare cell of the battery is increased by the width (W+W=2W) of the sealing parts formed on both sides of the pouch. Accordingly, when the width of the battery is fixed to a predetermined value, increasing a space in a widthwise direction of the battery to receive the electrode plates and electrolyte required to increase the capacity of the battery is relatively difficult. In addition, in the subsequent processes, the sharp corners may be easily damaged due to contact with an external part.